Method

Just a word first about preparing the vegetables. You need to use a potato peeler to pare off any really stringy bits from the outside stalks of the celery.

The nice thing is that the outside stalks are fine for soups – so if you're using a whole head of celery, once you've weighed out the amount you need, you can keep the tender inside stalks for munching on.

Peeling the celeriac may mean you lose quite a bit of the outside, as it's very fibrous. Once that's done, weigh it (you need 1 lb/450 g) and cut it into large chunks.

The celery should also be cut into large chunks, and the same with the onion.

All you do now is pop the whole lot into the casserole, then add the stock and bay leaves, along with some salt and freshly milled black pepper.

Bring it all up to simmering point on the hob, then put the lid on and transfer it to the oven to simmer very gently and slowly for 3 hours.

After that, remove the bay leaves, allow the soup to cool a little, then blend in batches until smooth. (A large bowl to put each batch in is helpful here.)

Then, return the soup to the casserole and bring it back to a gentle simmer, tasting to check seasoning before serving.

Serve in hot bowls with the yoghurt (or crème fraîche) spooned on top and the celery salt sprinkled over, garnished with a few celery leaves.